The term ionic liquids should be understood to mean salts or mixtures of salts whose melting point is below 100° C. (P. Wasserscheid, W. Keim, Angew. Chem. 2001, 112, 3926). Salts of this type known from the literature consists of anions, such as halogenostannates, halogenoaluminates, hexafluorophosphates or tetrafluoroborates combined with substituted ammonium cations, phosphonium cations, pyridinum cations or imidazolium cations to thereby form salts. Several publications have already described the use of ionic liquids as solvents for chemical reactions (T. Welton, Chem. Rev. 1999, 99, 2071, P. Wasserscheid, W. Keim, Angew. Chem., 2000, 112, 3926). For example, hydrogenation reactions of olefins with rhodium (I) (P. A. Z. Suarez, J. E. L. Dullius, S. Einloft, R. F. de Souza and J. Dupont, Polyhedron 15/7, 1996, 1217-1219), ruthenium (II) and cobalt (II) complexes (P. A. Z. Suarez, J. E. L. Dullius, S. Einloft, R. F. de Souza and J. Dupont, Inorganica Chimica Acta 255, 1997, 207-209) have been carried out successfully in ionic liquids with tetrafluoroborate anion. The hydroformylation of functionalized and non-functionalized olefins is possible with rhodium catalysts in ionic liquids with weakly coordinating anions (e.g. PF6−, BF4−) (Y. Chauvin, L. Mussmann, H. Olivier, European Patent, EP 776880, 1997; Y. Chauvin, L. Mussmann, H. Olivier, Angew. Chem., Int. Ed. Engl., 1995, 34, 2698; W. Keim, D. Vogt, H. Waffenschmidt, P. Wasserscheid, J. of Cat., 1999, 186, 481).
Further important fields of application of ionic liquids consists of their use as extraction agents for material separation (J. G. Huddleston, H. D. Willauer, R. P. Swatloski, A. E. Visser, R. D. Rogers, Chem. Commun. 1998, 1765-1766; b) A. E. Visser, R. P. Swatlowski, R. D. Rogers, Green Chemistry 2000, 2(1), 1-4) and of their use as heat carrier (M. L. Mutch, J. S. Wilkes, Proceedings of the Eleventh International Symposium on Molten Salts, P. C. Trulove, H. C. De Long, G. R. Stafford and S. Deki (Hrsg.), Proceedings Volume 98-11, The Electrochemical Society, Inc, Pennington, N.J.; 1998, page 254).
Even if the definition of an ionic liquid includes those salts whose melting point is between the room temperature and 100° C. it is still necessary and desirable for many applications for the ionic liquids to be liquid at temperatures below room temperature.
Numerous examples of such ionic liquids are known; however, as a rule these systems possess halide ions such as F−, Cl−, Br− or I− or those anions that contain halogen atoms. Typical representatives of the latter anions are—without any claim to completeness—(BF4)−, (PF6)−, (CF3COO)−, (CF3SO3)−, ((CF3SO2)2N)−, (AlCl4)−, (Al2Cl7)− or (SnCl3)−. The use of such anions containing halogen atoms imposes serious restrictions on the applicability of the corresponding ionic liquids: a) the use of these anions leads to considerable costs since even the alkali salts of these ions are very expensive; b) the hydrolysis products of these anions containing halogen atoms lead to considerable corrosion in steel reactors and in some instances also in glass reactors; and c) the thermal disposal of a “spent” ionic liquid with anions containing halogen atoms usually causes corrosion and environmental problems and is therefore very costly. The disposal via degradation in a biological clarification plant is also rendered difficult by the presence of anions containing halogen atoms.
In general, ionic liquids free from halogen atoms are therefore of particular interest since they possess the following properties:
a) a melting point and/or glass transition point of less than 25° C.;
b) hydrolytic-stability in neutral aqueous solution (pH=7) up to 80° C.;
c) disposal of by thermal means without the formation of problematic combustion gases;
d) degradability in biological clarification plants; and
e) commercially available of the anion as an alkali salt at a favorable price.
Among the ionic liquids free from halogen atoms according to the state of the art, there have been no representatives so far capable of satisfying this complex technical requirement profile. Thus nitrate melts, nitrite melts, sulfate melts (J. S. Wilkes, M. J. Zaworotko, J. Chem. Soc. Chem. Commun. 1992, 965) and benzene sulfonate melts (H. Waffenschmidt, Dissertation, RWTH Aachen 2000) are known, however, these ionic liquids have melting points above room temperature. Hydrogen sulfates and hydrogen phosphates react in aqueous solution while splitting off one or several protons and form acidic aqueous solutions. Methyl sulfate and ethyl sulfate melts exhibit a distinct hydrolysis after only 1 h at 80° C. in aqueous solution with the formation of hydrogen sulfate anions and the corresponding alcohol (compare also comparative examples 1 and 2).
Therefore, a need remains for ionic liquids that have a low melting point and possess other advantageous properties as described herein.